The field of invention relates to optical component technology generally; and, more specifically, to a laser fusion based WDM coupler.
A Wavelength Division Multiplexed (WDM) coupler module is a device that, through an arrangement of discrete WDM couplers, merges N optical channels onto a single optical fiber. FIG. 1 shows an embodiment of an 8xc3x971 WDM coupler module that is constructed with an arrangement of seven 2xc3x971 WDM couplers 1011 through 1017. The 8xc3x971 WDM coupler module of FIG. 1 is responsible for integrating 8 optical channels onto a single optical fiber. An optical channel corresponds to the optical permissiveness of a fiber optic path, as a function of wavelength, within an optical wavelength range that is referenced around axe2x80x9cpeakxe2x80x9d wavelength.
Optical permissiveness is figure of merit as to the tendency of a fiber optic path to allow light to propagate forward. Thus, if the optical permissiveness of a fiber optic path is xe2x80x9chighxe2x80x9d, the fiber optic path tends to allow light to propagate forward; and, if the optical permissiveness of a fiber optic path is xe2x80x9clowxe2x80x9d, the fiber optic path tends to xe2x80x9cblockxe2x80x9d light from propagating forward. Those of ordinary skill typically measure optical permissiveness for an optical device (such as a WDM coupler module) by measuring the intensity of light received at an output as a function of wavelength. The curve that is xe2x80x9ctraced outxe2x80x9d is often referred to as the xe2x80x9cspectrumxe2x80x9d of the optical path being measured (and which is being referred to herein as optical permissiveness).
Typically, the optical permissiveness of an optical channel within a WDM coupler module xe2x80x9crolls offxe2x80x9d as the wavelength deviates from its associated peak wavelength. As such, it may be said that the shape of an optical channel rolls off as optical wavelength deviates from its peak wavelength. FIG. 1 demonstrates an example by way of a depiction 103 of the optical permissiveness of the 8xc3x971 WDM coupler module (as observed from its output 102). Note that eight unique optical channels are observed 1041 through 1048. Each of the optical channels 1041 through 1048 has its own corresponding peak wavelength xcex1 through xcex8, respectively. Note that, from their shape, each of the optical channels 1041 through 1048 tend to pass light having its corresponding peak wavelength and tend to increasingly reject or block light that deviates from its peak wavelength.
As can be seen from FIG. 1, the 8xc3x971 WDM coupler module is formed with seven 2xc3x971 WDM couplers 1011 through 1017. Here, each 2xc3x971 coupler integrates onto its output fiber the light intensity that is received from its pair of input fibers. For example, 2xc3x971 coupler 1011 is generally designed to receive light intensity (at a first fiber optic input) that peaks at wavelength xcex1 and receive light intensity (at a second input) that peaks at wavelength xcex5. The 2xc3x971 WDM coupler 1011 integrates the received light intensity onto its output optical fiber (which also acts as a first input to 2xc3x971 coupler 1015). As such, the notation xe2x80x9cxcex1,xcex5xe2x80x9d is used proximate to the output of 2xc3x971 coupler 1011.
By nature of the specific combinations of input wavelength observed in the 8xc3x971 WDM coupler module of FIG. 1, note that each successive 2xc3x971 coupler (passing forward through the coupler module) may be designed with decreased spacing between neighboring optical channels. For example, the 8xc3x971 WDM coupler module of FIG. 1 may be designed such that: 1) 2xc3x971 WDM couplers 1011 through 1014 each have a neighboring channel peak wavelength spacing of 4(xcex8xe2x88x92xcex1)/7; 2) 2xc3x971 WDM couplers 1015 and 1016 each have a neighboring channel peak wavelength spacing of 2(xcex8xe2x88x92xcex1)/7; and 3) 2xc3x971 WDM coupler 1017 has a neighboring channel center wavelength spacing of (xcex8xe2x88x92xcex1)/7.
FIGS. 2a through 2c relate to the construction of a 2xc3x971 coupler. FIG. 2a shows a cross section of a typical optical fiber. The optical fiber cross section of FIG. 2a shows a central core 201 surrounded by a cladding layer 202. A protective jacket 203 surrounds the cladding layer 202. A common embodiment further includes a core 201 diameter of 5-9 xcexcm and a cladding layer 202 diameter of 125 xcexcm. FIG. 2b shows an initial manufacturing xe2x80x9csetupxe2x80x9d just prior to manufacture of a 2xc3x971 WDM coupler. According to the depiction of FIG. 2b a pair of optical fibers which have been stripped of their corresponding jackets are fixedly positioned next to one another. Here, FIG. 2b shows the cladding layer 212 and central core 211 of a first optical fiber; and, the cladding layer 222 and central core 221 of a second optical fiber.
Within a fusion region 230, the pair of stripped optical fibers neighbor one another. Heat is then applied within the fusion region 230 through the use of an open flame. As a consequence of the extreme heat that is applied to the fusion region 230, the pair of optical fibers begin to fuse together. FIG. 2b shows a depiction of the pair of optical fibers after they have been fused together (e.g., after the open flame has been removed). Because of the merging of the fibers, a 2xc3x971 coupler can be readily formed. For example, optical fiber end 231 can be viewed as the output of the 2xc3x971 coupler, optical fiber end 211 can be viewed as a first input to the 2xc3x971 coupler, and optical fiber end 221 can be viewed as a second input to the 2xc3x971 coupler. Section 232 can be terminated as xe2x80x9cno functionxe2x80x9d port.
Note that the cores from the pair of optical fibers are merged in the depiction of FIG. 2c. Typically, couplers requiring a narrow neighboring channel spacing (e.g., such as coupler 1017 of FIG. 7) may need to have merger of the cores within the fusion region in order to obtain the narrow channel spacing. Couplers having a more relaxed neighboring channel design (e.g., such as couplers 1011 through 1014 of FIG. 7) may be able to allow some degree of separation of the fiber optic cores.
FIGS. 3a and 3b relate to a traditional problem involved in the manufacture of WDM couplers. FIG. 3a shows optical permissiveness as a function of wavelength for a typical taper of optical fiber made by a flame fusion process. For any type of optical fiber made by flame fusion, a defect (that is related to the water absorption introduced by a traditional flame fusion process) causes a noticeable and undesirable xe2x80x9cbumpxe2x80x9d 301 in the optical permissiveness of the optical fiber taper (approximately over a wavelength range of 1370 nm to 1420 nm.
The bump 301 has two drawbacks. Firstly, the drop corresponds to increased xe2x80x9cinsertion lossxe2x80x9d of optical devices (such as WDM couplers and coupler modules) that process light having wavelengths in the realm of the bump 301; and, secondly, such insertion loss varies in the realm of the bump 301. As increased insertion loss corresponds to more optical rejectionxe2x80x94increased insertion loss by itself may threaten the practical use of an optical device (because most optical networks attempt to minimize the insertion loss caused by its various components). Moreover, many optical devices are designed to have substantially even (or xe2x80x9cflatxe2x80x9d) optical permissiveness over the range of used optical wavelengths (each peak wavelength for the optical channels of a WDM coupler module). The bump 301 corresponds to a deviation from this desired property.
FIG. 3b illustrates the combined effect of both drawbacks for a WDM coupler module. FIG. 3b (which may be compared with the optical permissiveness 103 of FIG. 1) corresponds to the optical permissiveness of an 8xc3x971 coupler that is made from 2xc3x971 couplers having fiber optic properties that suffer from water absorption. Assuming that the 8xc3x971 coupler is designed to operate over the wavelength range that is impacted by the water absorption xe2x80x9cbumpxe2x80x9d of FIG. 3b (e.g., xcex1=1380 nm; xcex2=1385 nm; xcex3=1390 nm; xcex4=1395 nm; xcex5=1400 nm; xcex6=1405 nm; xcex7=1410 nm; xcex8=1415 nm), each of the optical channels 3041 through 3047 suffer additional loss in permissiveness and suffer different degrees of loss in permissiveness because of water absorption.
As a consequence, varying peak wavelength permissiveness is observed across the range of optical channels 3041 through 3047xe2x80x94some of which may be sufficiently severe (e.g., the insertion loss 302 for optical channel 3044) so as to cause an optical channel to fail to meet a minimum required permissiveness. Moreover, it is important to note that even though optical fibers that are xe2x80x9cfreexe2x80x9d of water absorption problems are availablexe2x80x94their use does not remove the water absorption problem with respect to the manufacture of WDM couplers. Better said, even if one uses water absorption xe2x80x9cfreexe2x80x9d optical fibers during the manufacture of a WDM coupler, water absorption related xe2x80x9cproblemsxe2x80x9d still arise. Here, the open flame fusion process is believed to re-introduce the fibers to a water absorption susceptibility state. As a consequence, use of optical fibers that are initially free of water absorption does not significantly remove the ill-effects of water absorption with respect to manufactured WDM couplers.